1. Field of the Invention
The present invention relates to a single crystal pulling apparatus used when growing a single crystal in accordance with bringing a seed crystal in contact with a melt, and then pulling this seed crystal from the melt, and a droppage preventing device for preventing droppage when pulling a single crystal.
2. Description of the Related Art
In line with the progress of VLSI in recent years, there has arisen a need to lower costs during the VLSI fabrication. To realize this cost reduction, it is imperative that the diameter of the silicon wafer be made larger. In order to efficiently obtain large diameter wafers in large numbers, it has been learned that when pulling and growing a single crystal in accordance with the so-called Czochralski process (CZ process), the larger the crystal diameter, the longer the semiconductor silicon single crystal should be.
However, this is premised on the pulling and growing of a dislocation-free good quality single crystal.
For this reason, prior to enlarging the silicon single crystal from the seed crystal to the desired crystal diameter, ordinarily there is performed an operation called necking, by which the diameter of the crystal grown from the seed crystal is narrowed one time.
Accordingly, to realize a dislocation-free, there is an upper limit to the thickness of the necking, that is, the thickness of the Dash's neck portion, and, from the standpoint of tensile strength, this poses a problem in pulling a single crystal having a Dash's neck portion diameter of the thickness of the upper limit thereof.
In other words, when pulling a large diameter, heavy silicon single crystal, there is the concern that the Dash's neck portion (hereinafter called the neck portion for convenience sake), which has a thin diameter and insufficient mechanical strength, will break, making it impossible to achieve the anticipated goal of pulling and growing a single crystal.
Accordingly, to avoid this kind of situation, there has been proposed a method, which, as shown in FIG. 1, forms a nodule portion below the neck portion 7 of a single crystal 6, forms a neck-shaped mating portion 8, which has sufficient mechanical strength, and joins this mating portion 8 so as to mate with a mating member provided at the tip of an arm as this arm is pulled up.
In this case, at first, the arm mating member is at standby at a location separated from the surface of the melt inside the furnace, a seed crystal at the tip of a wire is brought in contact with the melt, a neck portion is formed, then, at the point in time when the mating portion is formed, the arm changes attitude so that the arm mating member mates with mating portion of the single crystal. After the single crystal is mated with the arm in this way, while the single crystal is being held by the arm, the arm is raised together with the above-mentioned wire, and the single crystal is pulled.
A device of this types was disclosed in Japanese Patent Application Laid-open No. 9-2893.
In this publicly-disclosed invention, a pair of mutually intersecting arms is used. The pair of arms are closed in accordance with an actuator, and the single crystal is sandwiched between and held by the mating pawls at the tips of the arms.
However, with this pair of intersecting arms, if force is not applied to the tips of the arms by an actuator, they are not able to sandwich and hold the single crystal.
That is, for example, if the operation of the actuator is canceled and the arms are raised, a bending moment acts on the arms in a direction that causes the arms to open. Consequently, the single crystal must be sandwiched and held in accordance with the mating pawls at the tips of the arms by applying via an actuator an opposite bending moment capable of overcoming this bending moment.
In this manner, the shape of a conventional arm constitutes a shape, by which the mating pawl of the arm is not maintained in an attitude that enables mating with the mating portion of the single crystal in accordance with the self-weight of the arm itself (if force is not applied by an actuator, the arm opens), and to raise the arms while maintaining as-is the mating state of the single crystal, a force for causing mating with the arms (a force for preventing the opening of the arms) must be transmitted at all times.
Therefore, in this manner, in line with the operation of the actuator, it is necessary to increase the mechanical strength of the arm in order to cope with the force applied to the arm, that is, the bending moment that acts on the arm, so that the arm member must be large and thick. Consequently, the arms weigh more, and also take up more space, and in addition to increasing arm manufacturing costs, give rise to the problem of having to make the pulling apparatus larger.
Further, because a device of the type that sandwiches and holds a single crystal with a pair of intersecting arms of this kind constitutes a structure which sandwiches and holds a single crystal from two sides, often times the arm tips open easily in line with the raising of the arms, and the single crystal separates from the mating pawls.
Accordingly, the invention publicly disclosed in Japanese Patent Application Laid-open No. 9-183694 is constituted so as to provide, separate from the mating pawls at the tips of the arms, an arm-latching mechanism to prevent the arm tips from opening. In other words, it is constituted so as to provide an arm-latching mechanism inside the furnace.
However, the inside of the furnace is hot, and in a near vacuum state, and providing a complex mechanism such as this arm-latching mechanism thereinside is problematic from the standpoints of the reliability and durability of the mechanism.
Moreover, since the inside of the furnace is the place where a single crystal is grown, the inside thereof must be maintained in an environmentally clean state.
Accordingly, providing a mechanism, such as the above-mentioned arm-latching mechanism, which raises concerns about the dust generated in line with operation means there are concerns that an environmentally clean state cannot be maintained.
As described above, a conventional single crystal-mating arm constitutes a shape, which is unable to maintain the single crystal mating state when the arm is being raised unless a force is constantly applied by an actuator.
Consequently, to cope with the bending moment, which acts on the arm in line with the operation of the actuator, it was necessary to enhance the mechanical strength of the arm, thus increasing arm manufacturing-related costs, and furthermore, increasing the size of the arm, making it impossible to make the pulling apparatus more compact.
Further, the mating pawl at the tip of a conventional arm constitutes a shape, which causes the single crystal to drop down (arm tip opens) when the arm is raised as-is.
Consequently, to prevent single crystal droppage (arm opening) when raising the arm, it was necessary to provide a separate arm opening preventing mechanism to prevent arm opening, and the dust generated in line with the operation of this arm opening preventing mechanism can have an adverse affect on the clean environment during single crystal growth. Further, since it is necessary to provide a complex mechanism inside the furnace, problems arise from the standpoints of reliability and durability as well.